An internal combustion engine for a vehicle may operate in a variety of combustion modes. One example mode is spark ignition (SI), where a spark performed by a sparking device is used to initiate combustion of an air and fuel mixture. Another example mode is homogeneous charge compression ignition (HCCI), where an air and fuel mixture achieves a temperature where autoignition occurs without requiring a spark from a sparking device. In some conditions, HCCI may have greater fuel efficiency and reduced NOx production compared to SI. However, in some conditions, such as with high or low engine loads, it may be difficult to achieve reliable HCCI combustion. Thus, depending on operating conditions, the engine may use either SI and/or HCCI combustion.
Another issue with regard to HCCI combustion is a potential for increased NVH due to a more rapid energy release rate from the faster combustion, thereby generating an increased pressure rise rate and an increased peak pressure. The higher HCCI pressure rise rate may extenuate the engine's normal Out of Balance Secondary Forces (OoBSF). OoBSF develop as the linear travel of the reciprocating piston and connecting rods transfers into crankshaft rotational movement. As a result, the piston may not follow simple harmonic motion. Rather, starting at top dead center (TDC) of piston position, the piston travels over half the bore permissible travel during the crankshaft's first 90 crank angle degrees (CAD). Then, during the next two sets of 90 CAD, the piston travels a reduced distance. Finally, during the last 90 CAD (270 to 360 degrees) the piston covers over half the remaining permissible travel once more completing the cycle. Therefore the piston speed during one complete revolution, starting at TDC, can be referred to as fast-slow-slow-fast. As the piston velocity changes, the OoBSF can affect driver feel, NVH and accelerated component wear within the engine. Thus, the higher HCCI pressure rise rate occurring during the natural piston velocity change may exaggerate the OoBSF effects. Furthermore, there may be significant change in the NVH observed into or out of HCCI mode during a mode switch.
While balance shafts may be used to address the NVH issues of HCCI combustion, use of balance shafts during SI mode may impair fuel economy and increase cylinder torque variations.
One approach to overcoming the above disadvantage may include a method of operating an engine having an adjustable balance shaft that includes adjusting operation of the balance shaft during engine operation in response to an engine combustion mode. For example, adjusting the balance shaft when switching may be used to improve the NVH differential between modes, and thus reduce driver perception of the change. As another example, adjusting the balance shaft differently for different combustion modes may be used to improve the NVH in both modes by taking into account the different combustion characteristics, and their effect on OoBSF, while also addressing fuel economy concerns. E.g., balance shaft operation may be disabled under conditions where the NVH effects are not present, such as during SI mode, thus improving fuel economy by reducing friction losses of the balance shafts.
The inventors herein have recognized the above issues and approaches. Still further issues, examples, and advantages are described herein.